1. Field of the Invention
The present invention relates to a non-oriented electrical steel sheet, which is used as an iron core material of an electrical apparatus, having unprecedentedly excellent magnetic properties such as exceedingly high magnetic flux density and low core loss, excellent formability such as excellent punching property, and excellent rust resistance to a product manufactured by using said non-oriented electrical steel sheet and to a production method thereof.
2. Description of the Related Art
In recent years, movements for improving efficiency are rapidly spreading in the field of electrical machinery and apparatuses, specifically rotating machinery and medium- and small-sized transformers, where non-oriented electrical steel sheets are used as iron core materials, amid the worldwide movement for the global environmental preservation, including the saving of electric power and energy and regulations against freon gas emission. For this reason, demands for improving the properties of non-oriented electrical steel sheets, namely, for higher magnetic flux density and lower core loss, are growing stronger.
The core loss reduction of a non-oriented electrical steel sheet has been carried out mainly by increasing the electrical resistivity through the addition of Si and Al and, by doing so, reducing the Joule heat loss caused by the loss of the eddy current that flows through each steel sheet constituting an iron core during its service.
However, among the energy losses of a rotating machine or an apparatus containing an iron core, the energy loss shared by copper loss, which is the Joule heat loss caused by the electric current flowing through a coiled wire wound round the core, cannot be neglected. In order to reduce the copper loss, it is effective to reduce the current density required to excite a core to a certain magnetic field strength, and therefore, the development of a material that exhibits a higher magnetic flux density with a same exciting current cannot be avoided. Namely, the development of a non-oriented electrical steel sheet having ultra-high magnetic flux density is essential.
By realizing a non-oriented electrical steel sheet having ultra-high magnetic flux density, it becomes possible to miniaturize both a rotating machine and an iron core and, for a movable body like an automobile or an electric car where a rotating machine and an iron core are mounted, it also becomes possible to reduce the energy loss during operation by the weight reduction of the whole body. Further, in case of a rotating machine, the torque is increased and a smaller-sized and higher-power rotating machine can be realized.
Thus, if a non-oriented electrical steel sheet having ultra-high magnetic flux density can be realized, not only the energy loss of an iron core and a rotating machine during their operation can be reduced, but also the pervasive effect inestimably extends to the entire equipment system incorporating them.
Conventional production methods of non-oriented electrical steel sheets having high magnetic flux density will be outlined. In Japanese Examined Patent Publication No. S62-61644, disclosed is a method of coarsening a crystal structure after hot-rolling by controlling the hot-rolling finishing temperature to 1000xc2x0 C. or more, and also coarsening the crystal structure before cold-rolling while eliminating a finish-annealing process. However, in an actual finish hot rolling mill, there is a disadvantage of the difficulty in eliminating the uneven temperature distribution along the longitudinal direction of a steel coil and thus the magnetic properties varying along the longitudinal direction thereof, because the rolling speed at the time when rolls bite the tip of the steel coil is different from the one under a steady rolling state.
In the mean time, in Japanese Unexamined Patent Publication Nos. S54-76422 and S58-136718, disclosed is a method of self-annealing by coiling a hot-rolled steel sheet at a high temperature between 700xc2x0 C. and 1000xc2x0 C. and annealing the coil itself with the heat retained therein as a means to suppress a cost increase caused by the addition of a process for annealing the hot-rolled steel sheet and to coarsen the crystal structure before cold-rolling. In the embodiments of these patent publications, however, all of the self-annealing are carried out in the xcex1-phase region for an identical reason, and the coarsening of the crystal structure before cold-rolling is limited.
Further, in Japanese Examined Patent Publication No. H8-32927, disclosed is a technology of pickling a hot-rolled steel sheet consisting of a steel material containing less than 0.01% of C, 0.5% to 3.0% of Si, 0.1% to 1.5% of Mn, 0.1% to 1.0% of Al, 0.005% to 0.016% of P and less than 0.005% of S, thereafter cold-rolling the pickled sheet at a cold reduction ratio of 5% to 20%, annealing the cold-rolled sheet for 0.5 to 10 minutes at a temperature between 850xc2x0 C. and 1000xc2x0 C., or for 1 to 10 hours at a temperature between 750xc2x0 C. and 850xc2x0 C., and then applying finish-annealing. This method is insufficient in improving magnetic flux density as compared to the conventional hot-rolled steel sheet annealing method and cannot meet the customers"" demands for improving the magnetic properties of a non-oriented electrical steel sheet.
In addition, as the methods of improving the magnetic properties of non-oriented electrical steel sheets by improving the primarily re-crystallized texture, disclosed are the methods of manufacturing non-oriented electrical steel sheets excellent in magnetic properties by improving the texture with the addition of Sn in Japanese Unexamined Patent Publication No. S55-158252, Sn and Cu in Japanese Unexamined Patent Publication No. S62-180014, or Sb in Japanese Unexamined Patent Publication No. S59-100217.
However, even the addition of these texture controlling elements, like Sn, Cu or Sb, cannot satisfy the customers demands for a non-oriented electrical steel sheet having ultra-high magnetic flux density and low core loss.
As another method, the improvement in the production process such as devising a finish-annealing heat cycle is implemented as disclosed in Japanese Unexamined Patent Publication S57-35626. However, the attempts reveal little effect on the magnetic flux density improvement, though core loss improvement is seen.
There are three known technologies for obtaining high magnetic flux density by adding Ni, as described below.
In Japanese Unexamined Patent Publication No. H6-271996, disclosed is a method of obtaining high magnetic flux density and low core loss by adding the elements of Sn, Sb, Cu and the like in addition to Ni. However, in actual production, there is a problem of increasing production cost since it is required to control the cooling rate in the two-phase region from the Ar3 point to the Ar1 point either after solidification by rapid cooling or by heating the material again to a temperature not less than the AC3 transformation temperature after the rapid cooling. Further, in Japanese Unexamined Patent Publication No. H8-246108, disclosed is a material having high magnetic flux density and low anisotropy realized by the addition of Ni. However, in actual production, it is required to finish-anneal the material by heating it to a temperature not less than the AC3 temperature, and therefore, there is a problem of easily deteriorating the core loss on account of the internal oxidation of the Ni-added steel. In addition, in Japanese Unexamined Patent Publication No. H8-109449, disclosed are a material claiming to have high magnetic flux density and low anisotropy by adding Ni and its production method. However, in the actual production method, the annealing of a hot-rolled steel sheet or the self annealing of the same is essential, and the problem of easily deteriorating the core loss on account of the occurrence of the internal oxidization of Ni during the annealing cannot be solved.
As described above, the conventional technologies can not produce a non-oriented electrical steel sheet having not only low core loss but also ultra-high magnetic flux density, and therefore can not satisfy the above-described demands for a non-oriented electrical steel sheet.
The present invention is characterized not only by furnishing an Ni-added steel with ultra-high magnetic flux density, but also by offering a low cost process capable of achieving ultra-high magnetic flux density and low anisotropy without requiring any particular heat treatment, and this feature can be attained by reducing the amounts of added alloys except Ni and adding P. Further, the internal oxidization of Ni can be prevented by applying finish-annealing at a low temperature in the xcex1-phase region, and by doing so, it becomes possible to make B25, which is the magnetic flux density at the magnetic field strength of 2500 A/m and is lower than B50, to 1.70T or higher, and at the same time, to make B25R, which is the magnetic flux density calculated by the equation (2), to 1.65T or higher for the first time.
In the present invention, the addition of Ni and the control of the addition of Si, Al and Mn can remarkably enhance the marine weather resistance against sodium chloride and the like, in particular, by making dense the inner layer portions of the rust layers in the steel sheet surface layers and thus by suppressing the intrusion of chloride ions. Further, it has also become clear that the addition of P in an appropriate amount can further enhance the rust resistance which has been brought forth by the addition of Ni.
In addition, in the present invention, it is newly found that Nb which has been added in the conventional weather resistant steel remarkably deteriorates the magnetic flux density of a non-oriented electrical steel sheet, and by controlling the addition amount of Nb, a non-oriented electrical steel sheet with ultra-high magnetic flux density having rust resistance, weather resistance and magnetic properties together can be successfully developed.
Thanks to the above development, a non-oriented electrical steel sheet having ultra-high magnetic flux density according to the present invention can be processed and stored even in a plant and the like located in the environments near a seashore which have been inappropriate for the processing of a conventional non-oriented electrical steel sheet. At the same time, rusting during transportation can also be prevented and that is an advantage in simplifying the packaging.
Furthermore, in case of a magnetic switch core, the rust resistance of the bare surface of a metal is important since the end face of a switch is subject to an impact every time the switch operates, and therefore, required is a measure such as enclosing the switch itself in a special casing in the environment where the switch is likely to be exposed to sodium chloride and the like. However, by employing a non-oriented electrical steel sheet having ultra-high magnetic density and rust resistance according to the present invention, it becomes possible to use a magnetic switch in a corrosive environment where it has hardly been used so far.
Further, by employing a non-oriented electrical steel having ultra-high magnetic flux density and rust resistance according to the present invention, a magnetic switch can be miniaturized and the attractive force is also enhanced since a strong attractive force can be obtained by the effect of the ultra-high magnetic flux density even if the exciting current or the number of the windings of a wire is reduced.
The object of the present invention is to solve the problems of the conventional technologies and to provide a non-oriented electrical steel sheet having ultra-high magnetic flux density and low core loss.
The gist of the present invention is as follows:
(1) A non-oriented electrical steel sheet having ultra-high magnetic flux density, characterized by:
comprising a steel containing, in terms of wt %,
Si: 0.4% or less,
Ni: 2.0% to 6.0%,
Mn: 0.5% or less, and
P: 0.01% to 0.2%,
with the balance consisting of Fe and unavoidable impurities; and having a magnetic flux density B25 of 1.70T or higher and a magnetic flux density B50 of 1.80T or higher.
(2) A non-oriented electrical steel sheet having ultra-high magnetic flux density and low magnetic anisotropy, characterized by:
comprising a steel containing, in terms of wt %,
Si: 0.4% or less,
Ni: 2.0% to 6.0%,
Mn: 0.5% or less, and
P: 0.01% to 0.2%,
with the balance consisting of Fe and unavoidable impurities; having a magnetic flux density B25 of 1.70T or higher and a magnetic flux density B50 of 1.80T or higher; and the difference between the magnetic flux density B50L measured merely for a sample in the longitudinal direction and the magnetic flux density B50C measured merely for a sample in the cross direction being 350 Gauss or less.
(3) A non-oriented electrical steel sheet having ultra-high magnetic flux density and low core loss, characterized by:
comprising a steel containing, in terms of wt %,
Si: 0.4% or less,
Ni: 2.0% to 6.0%,
Mn: 0.5% or less,
P: 0.01% to 0.2%,
and further,
C: 0.003% or less,
S: 0.003% or less,
N: 0.003% or less, and
Ti+S+N: 0.005% or less,
with the balance consisting of Fe and unavoidable impurities; and having a magnetic flux density B25 of 1.70T or higher, a magnetic flux density B50 of 1.80T or higher, and a core loss W15/50 after pickling, cold-rolling and annealing of 8W/kg or less.
(4) A non-oriented electrical steel sheet having ultra-high magnetic flux density according to any one of the items (1) to (3), characterized by having a magnetic flux density B50 of 1.82T or higher.
(5) A non-oriented electrical steel sheet having ultra-high magnetic flux density, characterized by:
comprising a steel containing, in terms of wt %,
Si: 0.4% or less,
Al: 0.5% or less,
Ni: 2.0% to 6.0%,
Mn: 0.5% or less, and
P: 0.01% to 0.2%,
with the balance consisting of Fe and unavoidable impurities; and having a magnetic flux density B25R, defined by the undermentioned equation (1) of 1.65T or higher and a magnetic flux density B50R defined by the undermentioned equation (2) of 1.75T or higher,
B25R=(B25-L+2xc3x97B25-22.5+2xc3x97B25-45+2xc3x97B25-67.5+B25-C)/8xe2x80x83xe2x80x83(1), 
where
B25-L: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction of rolling.
B25-22.5: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction inclining at an angle of 22.5 degrees from the direction of rolling on a steel sheet surface.
B25-45: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction inclining at an angle of 45 degrees from the direction of rolling on a steel sheet surface.
B25-67.5: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction inclining at an angle of 67.5 degrees from the direction of rolling on a steel sheet surface.
B25-C: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction perpendicular to the direction of rolling on a steel sheet surface,
B50R=(B50-L+2xc3x97B50-22.5+2xc3x97B50-45+2xc3x97B50-67.5+B50-C)/8xe2x80x83xe2x80x83(2) 
where
B50-L: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction of rolling.
B50-22.5: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction inclining at an angle of 22.5 degrees from the direction of rolling on a steel sheet surface.
B50-45: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction inclining at an angle of 45 degrees from the direction of rolling on a steel sheet surface.
B50-67.5: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction inclining at an angle of 67.5 degrees from the direction of rolling on a steel sheet surface.
B50-C: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction perpendicular to the direction of rolling on a steel sheet surface.
(6) A non-oriented electrical steel sheet having ultra-high magnetic flux density and low core loss, characterized by:
comprising a steel containing, in terms of wt %,
Si: 0.4% or less,
Al: 0.5% or less,
Ni: 2.0% to 6.0%,
Mn: 0.5% or less,
P: 0.01% to 0.2%,
and further,
C: 0.003% or less,
S: 0.003% or less,
N: 0.003% or less, and
Ti+S+N: 0.005% or less,
with the balance consisting of Fe and unavoidable impurities; and having a magnetic flux density B25R defined by the undermentioned equation (1) of 1.65T or higher, a magnetic flux density B50R defined by the undermentioned equation (2) of 1.75T or higher, and a core loss W15/50 after pickling, cold-rolling and annealing of 8W/kg or less,
B25R=(B25-L+2xc3x97B25-22.5+2xc3x97B25-45+2xc3x97B25-67.5+B25-C)/8xe2x80x83xe2x80x83(1), 
where
B25-L: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction of rolling.
B25-22.5: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction inclining at an angle of 22.5 degrees from the direction of rolling on a steel sheet surface.
B25-45: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction inclining at an angle of 45 degrees from the direction of rolling on a steel sheet surface.
B25-67.5: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction inclining at an angle of 67.5 degrees from the direction of rolling on a steel sheet surface.
B25-C: magnetic flux density under the magnetic field strength of 2500 A/m, measured for a sample cut out in the direction perpendicular to the direction of rolling on a steel sheet surface,
B50R=(B50-L+2xc3x97B50-22.5+2xc3x97B50-45+2xc3x97B50-67.5+B50-C)/8xe2x80x83xe2x80x83(2), 
where
B50-L: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction of rolling.
B50-22.5: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction inclining at an angle of 22.5 degrees from the direction of rolling on a steel sheet surface.
B50-45: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction inclining at an angle of 45 degrees from the direction of rolling on a steel sheet surface.
B50-67.5: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction inclining at an angle of 67.5 degrees from the direction of rolling on a steel sheet surface.
B50-C: magnetic flux density under the magnetic field strength of 5000 A/m, measured for a sample cut out in the direction perpendicular to the direction of rolling on a steel sheet surface.
(7) A non-oriented electrical steel sheet having ultra-high magnetic flux density and low core loss according to the item (5) or (6), characterized by having the magnetic flux density B50R of 1.79T or higher.
(8) An iron core excellent in punching property used for any one of; a rotator and a stator of a rotating machine, a reactor, a ballast, a choke coil, an EI core and a transformer: characterized by manufactured using a non-oriented electrical steel sheet according to any one of the items (1) to (7).
(9) A magnetic shielding apparatus characterized by manufactured using a non-oriented electrical steel sheet according to any one of the items (1) to (7).
(10) A non-oriented electrical steel sheet having ultra-high magnetic flux density and composed of a just cubic texture, characterized in that the strength standardized at the locations of xcex1=90xc2x0, xcex2=90xc2x0 and 270xc2x0 in the (100) complete pole figure of the layer located in the center of the sheet thickness is 0.5 or higher.
(11) A non-oriented electrical steel sheet having ultra-high magnetic flux density and composed of just a cubic texture, characterized in that the strength standardized at the locations of xcex1=90xc2x0, xcex2=90xc2x0 and 270xc2x0 in the (100) complete pole figure of the layer located at the depth of one fifth of the sheet thickness from the surface is 0.5 or higher.
(12) A production method of a non-oriented electrical steel sheet having ultra-high magnetic flux density characterized by: using a slab containing chemical components specified in any one of the items (1), (2), (3), (5) and (6), with the balance consisting of Fe and unavoidable impurities; hot-rolling said slab to a hot-rolled steel sheet; cold-rolling said steel sheet once after pickling; and then applying finish-annealing.
(13) A production method of a non-oriented electrical steel sheet having ultra-high magnetic flux density according to the item (12), characterized by applying the finish-annealing in the xcex1-phase region.
(14) A non-oriented electrical steel sheet having ultra-high magnetic flux density, excellent rust resistance and excellent weather resistance according to any one of the items (1) to (7), characterized in that the content of Nb is less than 0.005 wt %.
(15) An iron core for a magnet switch excellent in rust resistance and weather resistance, characterized by manufactured using either a non-oriented electrical steel sheet according to the item (10) or (11) having the Nb content of less than 0.005 wt % or a non-oriented electrical steel sheet according to the item (14).